1. Field of the Invention
The present invention relates to an electrostatic-actuation type wavelength-tunable optical filter making use of the principle of a Fabry-Perot interferometer, and more particularly it relates to a wavelength-tunable optical filter which can be actuated at low voltage.
2. Description of the Related Art
FIGS. 7(a)–(b) are partial sectional views of an example of an electrostatic-actuation type wavelength-tunable optical filter which has been used for a wavelength-division multiplex (WDM) optical communication-network, wherein FIG. 7(a) is a horizontal sectional view illustrating a movable plate 1 having a movable mirror (not shown) fixed thereto and a spring portion 3 connecting the movable plate 1 to a peripheral portion 2, and FIG. 7(b) is a vertical sectional view illustrating the movable plate 1, the peripheral portion 2, an insulating portion 4, and an electrode portion 5, these two members being disposed below the movable plate 1 and the peripheral portion 2. The movable plate 1 has the movable mirror fixed to the upper surface thereof. When a periodic voltage is applied between the movable plate 1 and the electrode portion 5, an electrostatic force is exerted on the movable plate 1. Thus, the movable plate 1 moves periodically in the vertical direction such that a balance between forces of the elastic spring portion 3 and the electrostatic force is maintained. The wavelength-tunable optical filter shown in FIG. 7 makes use of the principle of a Fabry-Perot interferometer, and the principle will be described in detail later.
Also, in some of known electrostatic-actuation type wavelength-tunable Fabry-Perot filters, a portion of the filter corresponding to the spring portion 3 shown in FIG. 7 is spirally bent, and a hinge (spring portion) is made more flexible so that a membrane (movable plate) can move smoothly (see FIG. 3 in U.S. Pat. No. 6,341,039, for example).
In the known electrostatic-actuation type wavelength-tunable optical filter shown in FIG. 7, if the distance between the movable plate 1 and the electrode portion 5 is made larger in order to achieve a large displacement of the movable plate 1, since the electrostatic force is in inverse proportion to the square of the distance between the movable plate 1 and the electrode portion 5, the voltage to be applied must be increased. On the other hand, if the distance between the movable plate 1 and the electrode portion 5 is made smaller in order to prevent an increase in the voltage to be applied, the displacement of the movable plate 1 is limited since it hits against the electrode portion 5 when moving downward. Hence, a low-voltage actuation and a large displacement cannot be achieved at the same time.
Also, when a sharply changing voltage such as a pulse voltage is applied between the movable plate 1 and the electrode portion 5, the movable plate 1 moves beyond a desired position, that is, causes a problem of a so-called overshooting is caused.
In addition, in the known electrostatic-actuation type wavelength-tunable Fabry-Perot filter (see the foregoing patent document, for example), a low-voltage actuation and a large displacement of a membrane (movable plate) cannot be achieved at the same time similar to the wavelength-tunable optical filter shown in FIG. 7, and a problem of an overshooting of the membrane occurs.